In a number of applications, high speed, small size, and low radiation error are required from measurement detectors, in particular from detectors intended for measurement of temperature or relative humidity. Said requirements are particularly strict, e.g., in detectors of radiosondes.
As is known in prior art, for example, as temperature detectors of radiosondes, as a rule, capacitive detectors are used, whose active material is a ceramic whose dielectricity is dependent on the temperature. The prior-art glass-ceramic temperature detectors are, however, of relatively large size, and therefore their speed and radiation error would require some improvement. The radiation error produced by solar radiation has been the most important problem in the temperature measurement by radiosondes with the use of prior-art temperature detectors.
Besides capacitive detectors, in radiosondes and equivalent, resistive temperature detectors and thermoelements have also been used.
In the prior art, capacitive humidity detectors are known in whose capacitance the dielectric material used is a polymer, a ceramic, or a glass-ceramic whose dielectric constant is a function of the humidity absorbed by it. The speed and corresponding properties of these detectors also require development, in particular in view of radiosonde applications.
As regards the prior-art electrically detected temperature and humidity detectors, reference is made, e.g., to the U.S. Pat. Nos. 3,168,829 and 3,350,941 as well as to the applicant's FI Patent No. 48,229, in which latter FI patent a capacitive humidity detector is described in which the dielectric insulating material is a polymer film whose permittivity is a function of the amount of water absorbed by the polymer film.
The substantial construction of the humidity detector described in said FI patent is a thin plate, onto whose face a structure sensitive to humidity has been processed. A drawback in this prior-art construction is the scattering of the permeability to water of the surface electrode and the resulting scattering of speed of the detector. Further, the size of the construction is quite large, which produces a radiation error especially in radiosonde operation, because the face subjected to solar radiation is considerable and its ventilation is relatively poor. Since, in said prior-art detector, the active face is straight, water tends to remain on this face as drops.
The processes of manufacture of the prior-art impedance detectors and equivalent have been complicated, frequently poorly suitable for automation and poorly suitable for continuous processes of manufacture. The prior-art processes of manufacture of said detectors have been demanding and consisted of a number of steps, in particular in an attempt to obtain a detector as rapid and accurate as possible, which usually requires very small size and precise dimensioning from the constructions and good control of the various parameters in the processes of manufacture.
With respect to the most recent development work carded out by the applicant, related to and closely connected with the present invention, reference is made to the following FI Patent Applications: No. 921449 (filed Apr. 1, 1992), No. 933701 (filed Aug. 23, 1993) and No. 933702 (filed Aug. 23, 1993). The constructions and methods described in said applications can, where applicable, also be used in combination with the present invention.
The object of the present invention is further development both of the constructions and of the processes of manufacture of the prior-art humidity detectors so as to avoid the drawbacks mentioned above and to achieve the objectives stated above and those that will come out later.
The process and the detector construction described in the applicant's FI Patent Application No. 921449 (filed Apr. 1, 1992, became public Oct. 4, 1993) (equivalent to U.S. Pat. Appl. Ser. No. 08/040,129 and to EP Application 93 850 046.9) are most closely related to the present invention. In said FI patent application, an electrical impedance detector is described, in which the active material between its electrodes is expressly a very thin thread-like glass or glass-ceramic fibre prepared by the glass-drawing technique. Said fibre is a glass-ceramic fibre whose drawing into fibreglass thread has been carried out in glassy form whereas the crystallization into the glass-ceramic form has been carried out in a heat treatment. In the glass-ceramic material the active constituent is crystalline barium-strontium titanate Ba.sub.x Sr.sub.1-x TiO.sub.3 or crystalline lead-strontium titanate Pb.sub.x Sr.sub.1-x TiO.sub.3, wherein x is in the range of 0 . . . 1, and which is placed in a glass matrix. The cross-sectional shape of the detector thread is substantially circular, and its diameter is of an order of 25 . . . 500 .mu.m.
The construction of the detector described in said FI Pat. Appl. 921449 is preferably coaxial, comprising a solid central electrode wire or an equivalent hollow electrode wire and a glass or glass-ceramic layer placed around said electrode, on which layer there is a hermetic glass layer, and on it an electrode layer and/or electrode fibres which have been applied in a separate step. Said detector may be composed of two detector fibre threads, which are joined together by means of a parallel joint, e.g. an adhesive joint, over a certain length. Further, in said FI patent application, a method is described for the manufacture of electrical impedance detectors, which method comprises a combination of the following steps:
a continuous detector-fibre thread of substantially circular section is manufactured by means of a glass-drawing technique in itself known out of a molten glass mix which has been alloyed with an additive or with additives that provide(s) the active material of the detector with suitable electrical properties; the detector-fibre thread is crystallized by means of heat treatment into a glass-ceramic form or its material is chosen or otherwise treated so that an active detector material is produced whose capacitance and/or resistance depend on the temperature or, in particular cases, on the mount of water absorbed by the active material; and, for individual detectors, said detector-fibre thread is cut-off into suitable pieces of detector-fibre thread, to which terminals are connected, and/or to whose electrodes, which have been provided at the thread-drawing stage, terminals are coupled or connected, between which terminals the impedance of the detector can be measured.
In the process of said FI Pat. Appl. 921449 described above, the drawing of the detector fibre thread is carried out by using a double-crucible process known in itself from the manufacture of optical fibres, in which process, in the inner crucible, molten core glass is used, to which strontium, barium and titanium oxide and/or an equivalent other additive have been added, and in the outer crucible molten glass material is used, such as aluminosilicate glass, from which a tubular hermetic outer layer is obtained onto the detector fibre thread. In the process of said FI patent application, into the molten detector fibre, an electrode wire is fed, or the inner electrode is passed into a glass-tube blank, which are then together drawn into a detector fibre thread, or first a hollow detector fibre thread is prepared and its interior is metallized afterwards. In said process, the detector fibre thread is coated with a conductive electrode layer by passing the fibre thread through a crucible with a hole in its bottom and through an oven, said crucible containing conductive paste as the coating paste. Onto the fibre thread, conductor patterns are applied by vapour-deposition or by a photolithographic method. In the process, from the continuous detector fibre thread, pieces of about 1 . . . 5 cm are cut off, which pieces are joined together side by side by means of a longitudinal joint, or detector wires are attached to both sides of a piece of detector fibre thread. In the process of said FI application, the electrode wire does not operate as a carrier or drawing wire in the process of manufacture, said carrier or drawing wire being expressly a fibreglass thread.